Conventionally, upon actualizing of electrostatic latent image formed on an image holder, well-known development using developer including non-magnetic toner and magnetic carrier is made.
In an image forming apparatus using 2-component developing as a kind of this method, 2-component developer is stirred with a developer stirring-transfer member (hereinbelow, simply “stirring member”) provided in a developer container, and through frictional charging, the developer is transferred toward a developing sleeve having a fixed magnet roller inside. The developer is further transferred onto the surface of the developing sleeve and supplied to an electrostatic latent image on a image holder, thereby a visible image is formed on the image holder.
In the 2-component developing method, since only toner can be resupplied from a separately-provided toner supply container, the life of the image forming apparatus can be extended. Further, this method is advantageous in terms of running cost.
FIG. 6 shows a cross-sectional view of a general 2-component developing unit. Reference numeral 160 denotes a developer container including 2-component developer, and 161, a developing sleeve as a developer holder. The developing sleeve 161 is a hollow metal sleeve including a magnet roller (not shown) as magnetic field generating means inside. A blade 162 is provided below the developing sleeve 161 in the proximity of the sleeve 161. The developer transferred in accordance with rotation of the developing sleeve 161 in an arrow direction is formed into a thin layer by the blade 162. Then, in a portion opposite to an electrostatic drum 1000, developing is performed in faithfully accordance with the electrostatic latent image on an electrostatic drum 1000.
In the developer container 160, a screw 163 is provided in approximately parallel to the developing sleeve 161, to stir and transfer the developer in the arrow direction. A screw 164 is provided on the opposite side to the developing sleeve 161. Further, a toner density sensor 165 is provided on a wall surface in the rear of the screw 164 (left side in the figure).
FIG. 7 is a cross-sectional view of a developing unit viewed from an upper position. The screws 163 and 164 are provided in approximately parallel to each other. The inside of the developing unit is partitioned with an inner wall 168 to prevent developer from being transferred between the screws 163 and 164. The developing unit has no inner wall at both ends in a lengthwise direction such that the developer can be transferred between the screws 163 and 164. As the screws 163 and 164 transfer the developer respectively in opposite directions, a circular path is formed in the developing container 160 to continuously circulate the developer.
The toner density sensor 165 is provided on the upstream side in the transfer direction on the screw 164, such that it can immediately detect the toner density, which has been lowered after use of toner in image formation. The developer used in image formation on the screw 163 side is sent to the screw 164 side by the above-described circulation, and the toner density is detected by the toner density sensor 165. Then, based on the result of detection, an appropriate amount of toner is resupplied from a toner supply mechanism 169 through a toner supply port 167 on the downstream side of the toner density sensor 165. Thus a predetermined toner density of the developer can be maintained, and a sufficient charging amount can be applied to the toner.
Conventionally, a method for determination of the life of such developing unit by detection of the number of printed sheets has been proposed (e.g., Japanese Published Unexamined Patent Application No. Hei 10-039693).
The determination of the life of the developing unit is important especially in a process cartridge type image forming apparatus. The “process cartridge type” means an arrangement in which constituent elements for a process acting on an electrophotographic photo conductor member, such as an electrostatic drum and a charging unit (including the developing unit), are integrated as a process cartridge attachable/detachable to/from the image forming apparatus main body.
When the life of the process cartridge has expired, it is arbitrarily replaced with a new cartridge, thereby an output image obtained from the image forming apparatus always has a predetermined level of quality. Accordingly, it is important to accurately grasp time of replacement of respective process constituent elements.
However, in the conventional image forming apparatus, a so-called “photographic fog” phenomenon occasionally occurs before expiration of a predetermined life.
It is found from the survey of this problem that photographic fog occurs due to a reduction of the toner charge amount. That is, the triboelectrical-charging capability of the carrier in the developer is extremely degraded before the number of printed sheets becomes the predicted number of sheets upon expiration of the life of the carrier.
The life of the carrier in the developer (use amount) is also influenced by the toner use amount. There is a difference between the life conventionally determined based on the amount of the carrier in the developer, i.e., the number of revolutions of the developing sleeve or the number of printed sheets indicating the number of revolutions of the screw to stir the developer, and the actual life of the carrier in the developer.